1. Field of the Invention
The present invention relates to a sealed semiconductor component, and more particularly to an improved structure for the inner connection terminal board in a sealed semiconductor device.
2. Discussion of the Related Art
The constructions of conventional examples of a sealed semiconductor device are shown in FIGS. 5 through 7.
FIG. 5 is a perspective view showing, for example, the construction of a sealed semiconductor device prior to sealing the device. The sealed semiconductor device thus shown is provided with an inner connection terminal board 21. FIG. 6 is a schematic structure of the semiconductor device after it has been sealed. FIG. 7 is a perspective view of an alternate example of a conventional inner connection terminal board 21, shown apart from the sealed semiconductor device.
In the construction of the device shown in FIG. 5, a semiconductor chip 1 such as a chip of power transistor, MOSFET or IGBT, includes a plurality of integrated and self-contained semiconductor elements into one body. The semiconductor chip 1, an inner connection terminal board 21, a control terminal board (not shown) etc. are loaded onto a copper clad wiring circuit board 2. The semiconductor chip 1 and the wiring circuit board 2 are connected by inner wires 3 composed of any preselected material such as aluminum.
Inner connection terminal board 21 is formed from a conductive metal plate and is typically soldered to the copper clad wiring circuit board 2 at a predetermined location. Inner connection terminal board 21 consists of a solderable, fixed inner terminal portion 22, and an intermediate, vertically protruding portion 23 having a bent portion 24 upwardly extending from the inner terminal portion 22. Bent portion 24 forms a U-shape in parallel with the direction of width of the board. An outer terminal portion 25 first extends horizontally from the intermediate protruding portion 23 and then upwardly so as to protrude from the sealed semiconductor device.
As shown in FIG. 6, the copper clad wiring circuit board is located on an insulating substrate 4. A metal base board 5 having a function of releasing heat holds the insulating substrate 4. A side wall 6 and the base plate 5 constitute the case. Copper clad wiring circuit board 2, semiconductor chip 1, and insulating substrate 4 (except outer terminal portion 25 of the inner connection terminal board 21) are embedded in a silicon gel layer 7 in the case consisting of the base board 5 and the side wall 6. A sealing resin layer 8 is filled and solidified on top of silicon gel layer 7. A resin block 9 is arranged such that outer terminal portion 25 of the inner connection terminal board 21 protrudes from it.
The sealed semiconductor device having the inner connection terminal board 21 constructed as described above has been used in a radiation-cooled state by securing the metal base plate 5 of the case to a radiation-cooling means such as an outer cooling fin. As is conventionally known, when the semiconductor chip 1 is subjected to an energizing operation, it generates heat in proportion to its power consumption. In general, the semiconductor chip is designed to normally operate under a temperature of less than 150.degree. C. Thus, the internal temperature of such a device may also reach approximately 150.degree. C.
The components of the respective portions of the device thermally expand because of the heat generated by the semiconductor chip 1 during the energizing operation. The heat causes thermal expansion stress which has a tendency to peel the inner terminal portion 22 of inner connection terminal board 21 off of the connecting portion of the copper clad wiring circuit board 2. When the energizing operation ceases, the temperature of the device returns to the ambient temperature (i.e., room temperature) whereby the respective components of the device return from an expanded state to an unexpanded state. The contraction to an unexpanded state generates stress which presses the inner terminal portion 22 to the connecting portion. Consequently, the repetition of expansion and contraction exerts stress on the soldered portion that electrically connects inner terminal portion 22 to the connecting portion of copper clad wiring circuit board 2. When this stress reaches a maximum, it is possible that the soldered portion can become separated from the connecting portion due to the generation of cracks on the soldered connection face. The connection between inner terminal portion 22 and copper clad wiring circuit board 2 is then broken which results in the failure of the device.
In the construction of the conventional sealed semiconductor device, to relieve the mechanical stress generated by the cyclic thermal expansion produced by the heat generated during the energizing operation of the device, the vertical bent portion 24 of a U-shape has been used. The bent portion 24 in the intermediate protruding portion 23 relieves and absorbs the cycle stress through the cushioning action of the vertical bent portion 24 of a U-shape.
Although the bent portion 24 of the U-shape absorbs and relieves the mechanical stress due to thermal expansion and contraction during the energizing operation of the device, other problems arise because the bent portion 24 of a U-shape extends upwardly from inner terminal portion 22. The U-shape of bent portion 24 also presents problems, because it is arranged in the vertical direction from the wiring circuit board 2, yet it is bent in a U-shape in the direction of width. The thickness b, which is a height of the U-shaped bent portion 24 to be embedded in the silicon gel layer 7, must be increased as shown in FIG. 6. Further, it is necessary to increase thickness c, which is the height of the U-shaped bent portion 24 from the wiring circuit board 2, in order to avoid contact between the inner wires (metal wires) 3 and the inner connection terminal board 21. As a result, the total thickness a of silicon gel layer 7 becomes so large that a substantial quantity of silicon gel must be used to fill the space.
Additionally, in order to relieve and absorb the mechanical stress, it is important to reduce the width of the board at the U-shaped bent portion 24 so as to effectively obtain the cushioning action desired. However, a conflict arises in that the internal inductance of the terminal board itself increases as the width of the board increases and the device cannot be used as a module that may accept a large current. To correct this problem it has been conventionally proposed that the internal inductance of wiring is decreased by forming a board portion 24a at the upper portions of bent portion 24, as shown in FIG. 7. However, it is difficult to fill the case 6 with silicon gel and sealing resin when the board portion 24a is present. Moreover, the cost increases as the weight of the parts increases.